feat: generate alt text in charts

[alexdln]

Alt text may be important for my users and systems. It's needed both for immediate access to content and for sharing later in an A11Y-friendly environment (like bluesky).

Content needs to be generated based on the chart. Localization is possible if we can localize the entire chart as well.

It's usually standard practice to add the ability to copy alt text in the options.

Services typically have a character limit of 1000-2000. Bluesky has 2000 characters, so I think it's worth sticking to it.